Transcript How Legumes Form Nodules - University of Queensland

ARC Centre of Excellence
for Integrative Legume
Research
How
Legumes
Make
Nodules
Lisette Pregelj – Education and
Outreach Manager
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Legume Nodules
• Legume nodules are
special plant organs
that house nitrogenfixing bacteria
called Rhizobium
• Nodules form on
Legume roots
• Nodules can also
form on stems in
some species
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Vascular Bundles
Nodule Cortex
Sclerenchyma
Rhizobium Filled
Cells
>25,000 per cell
Nodule Cross-Section
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Nodule Formation
1. Rhizobia attracted to
root
2. Rhizobia attach to
root hairs
3. Root hair curling
4. Infection thread
formation
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Nodule Formation
5. Root cortical cell
division
6. Rhizobia invade
cortical cells
7. The nodule grows
8. Fully functional
nodule
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1: Rhizobia attracted to root
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1. Rhizobia attracted to root
• Legume roots exude
flavonoids
(Soybean exudes the
isoflavone genistein)
• Rhizobia are attracted
to flavonoids
Petri dish
contains a
bacterial lawn
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Bacteria turn
blue when a
reporter gene
is switched
on by plant
exudates
(flavonoids)
2. Rhizobia attach to root hairs
Rhizobia
Root Hair
10 mm
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2. Rhizobia attach to root hairs
Deformed root
hairs
Curled root
hair
Vascular
bundle
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3. Root hair curling
50 mm
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4. Rhizobia infect root hair
Root Hair
Root Hair Curling
Rhizobia
Infection Thread
Infection Pocket
Legume Root
Vascular Bundle
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4. Rhizobia infect root hair
50 mm
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5. Root cortical cells divide
100 mm
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5. Root cortical cells divide
100 mm
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6. Rhizobia invade cortical cells
50 mm
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7. The nodule grows
2-4 mm
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8. Fully functional nodule
root
nodule
Rhizobia (green
marker)
100 mm
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Nodulation Timeline
Roots
Inoculation
(day 0)
4-6 dpi
1-2 days post
inoculation (dpi)
10 dpi
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4 dpi
3 weeks pi
Chemical Signals
• Nodulation involved chemical signals
• Released by both the Legume plant and the
Rhizobium bacteria
Flavones
Nod Factors
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Flavones and Isoflavones
• Released by Legume roots
• Signal Rhizobia in the soil that a Legume is
present and ready to nodulate
• Soybean releases Genistein, an isoflavone
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Flavones and Isoflavones
Flavonoids in
Root tip and
Thickened root
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Nod Factors
• Rhizobia in response to
flavones release Nod
factors
• Nod factors are sugars
with specific signalling
functions
• Nod factors signal to a
legume that Rhizobia
are present in the soil
and ready to live in
nodules
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Nod Factors
• Each Rhizobum species releases unique Nod factors
• All Nod factors have a concerved backbone
• Difference is in the decorations (R1 R2 R3 R4 R5) and
number (n) of sugar repeats
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Nod Factor Receptors
• Each legume species has receptors
specific to a unique Nod factor
• Therefore each legume species
nodulates with its own Rhizobium
• Soybean -Bradyrhizobium japonicum
• Lotus – Mesorhizobium loti
• General – Rhizobium NGR234
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CILR
• University of
Queensland
• Australian National
University
• University of
Melbourne
• University of
Newcastle
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CILR
ARC Centre of Excellence for Integrative
Legume Research
Contact Us
HQ: University of Queensland
Tel: 3365 3550
Email: [email protected]
[email protected]
Visit Our Website!
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Picture and Diagram Credits
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Uli Mathesius
Mark Kinkema
Peter Gresshoff
Dana Hoffmann
Michael Sheahan
Sureeporn (Ning) Nontachaiyapoom
Paul Scott
Brett Ferguson
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